A few weeks ago I transferred from AC to LH in FRA - 3 of us had 6 carry on items (1 rollaboard plus backpack) in total but we were prepared in FRA to gate check 3 of the items when going through transfer security. The only thing we got at security was a gruff bark from one of the guys to remove my mask. We boarded the LH flight with the extra carry on items taking up their own row (Embraer). So YMMV.